Given the aging of the U.S. population, identifying strategies to optimize disease prevention and promote successful aging has important public health significance. Although the health benefits of regular moderate to vigorous intensity physical activity (MVPA) are well established, the health benefits of light intensity PA, the changes in PA and sedentary behavior (SB) during adulthood, and the role of these changes in health promotion and disease prevention are largely unknown. With data from the Coronary Artery Risk Development in Young Adults (CARDIA) study, the overall goal of this proposal is to evaluate the 10-year changes in objectively- measured PA and SB from early (ages 38-50) to late (ages 48-60) midlife in relation to cardiovascular disease risk and markers of successful aging, including physical and cognitive function and leukocyte telomere length (a cellular marker of cardiovascular aging). Participants will be all those who attend the CARDIA Year 30 core exam (June 2015 to May 2016) and agree to the ancillary study procedures (n~2,928). The protocol will include: 1) seven days of accelerometry (ActiGraph wGT3X-BT); 2) physical performance tests; and 3) DNA extraction from stored buffy coats for assays of leukocyte telomere length. Year 30 cognitive function data will come from a separately funded ancillary study. In addition, 100 participants from each race/gender group (total n=400) will participate in a sub-study designed to validate the 400 meter walk physical performance test as a measure of aerobic capacity, using the duration of a symptom-limited graded exercise treadmill test as the gold standard. These same 400 participants will also participate in a calibration study to adjust estimates of PA and SB from the Actigraph 7164 (used at Year 20) to the Actigraph wGT3X-BT. To accomplish these goals, we propose the following specific aims: 1) Describe patterns of change in accelerometer-measured PA and SB over 10-years and determine if these patterns vary by race, gender, and other socio-demographic and psychosocial factors; 2) Investigate independent and interactive associations of declines, maintenance (high vs. low), and increases in objectively-measured PA and SB over 10-years with risk of cardiometabolic disease (obesity, hypertension, dyslipidemia, type 2 diabetes mellitus, subclinical atherosclerosis, and clinical endpoints) at Year 30; and 3) Investigate independent and interactive associations of declines, maintenance, and increases in objectively- measured PA and SB over 10-years on cellular aging (from Years 20 to 30) and, on Year 30 cognitive and physical functioning. This proposed work has important public health significance because CARDIA will be the first U.S. cardiovascular cohort study with repeat, objective measures of PA and SB, uniquely positioning this study to examine the effects of PA and SB change on relevant indicators of successful aging and disease risk within an initially young and healthy biracial study population that is now middle-aged and at increasing risk for onset of disease and disability.